Homogeneous Catalysts

In this section, ruthenium and iridium both single-metal and dinuclear complex will be described since most of the mechanistic knowledge for water oxidation catalysis comes from studies carried out with these metal complexes. However, this information can be extensible to the rest of catalysts based on other transition metals.^47,48

Ruthenium Molecular Complexes

Ruthenium is located in the second transition metal series of theperiodic table.

With an electronic configuration [Kr] 4d⁷5s¹, it gives access to the widest variety of oxidation states, from Ru⁸⁺to Ru^{2 –}, which correspond respectively to d⁰and d¹⁰electronic configurations. Coordinated with polypyridyl ligands, ruthenium complexes are usually stable at high oxidation states, showing an octahedral geometry, and their redox properties can be tuned. In general, organometallic complexes offer the possibility to analyze their activity as a function of their ligand coordination.^49,50

In 1982, the first complex capable of mediating the four proton-coupled electron transfer (PCET)⁵¹process to oxidize H₂O, was reported as the dinuclear µ-oxo-bridged ruthenium complexcis,cis-[(bpy)₂(H₂O)Ru(µ-O)Ru(H₂O)(bpy)₂]⁴⁺more commonly known as the "blue dimer" due to its characteristic blue color (Figure 1.5 [a]).^52,53This study proved that the hard multielectron oxidation of H₂O to O₂ was possible. First studies revealed a limited catalytic activity, with 13.2 turnover number (TON) and a4.2·10⁻³seg⁻¹of turnover frequency (TOF). The presence of theµ-oxo bridge allows a strong electronic coupling between the two metallic centers, that stabilizes high oxidation states by electronic delocalization. The mechanism proposed by Meyer group involved four stepwise PCET that give rise

1.4 - Water Oxidation Catalysts (WOC) 17 to high-valent Ru^V−Ru^Vintermediate.⁵⁴The main reason of its low activity is related to the instability of theµ-oxo bridge, which is cleaved into two nonactive monomeric ruthenium complexes. Different studies have achieved stable blue dimer derivatives by tunning organic ligand structures capable to connect strongly the two ruthenium centers in a close proximity.

Another well studied ruthenium complex [Ru₂(OH₂)₂(bpp)(tpy)₂]²⁺were Hbpp cor-responds to 2,2’-(1H-pyrazole-3,5-diyl)dipyridine ligand, was also reported (Figure 1.5 [b]),^55,56being the first dinuclear ruthenium complex lacking a [Ru−O−Ru]

scaffold capable to oxidize H₂O to O₂. In this complex, the two ruthenium centers are placed at a close distance, in acisorientation to one another, by the addition of a rigid pyrazole scaffold as a conjugated bridge between the two metal atoms.

Extensive both experimental and theoretical studies have demonstrated that [O−O] bond formation proceeds only through the (I2M) pathway. Compared to the blue dimer it is more than 3 times faster in similar conditions, but still shows a moderate catalytic performance, the maximum TON being 17.5 and the overall efficiency 70%. The absence of theµ-bridge avoids decomposition pathways, which was a limitation of the blue dimer complex. Nevertheless, the efficiency is still low due to the oxidation of the C−H group of the pyrazole scaffold in the bridging ligand. Further modifications adding an extra methyl group in the pyrazole moiety increases its performance.

18 Chapter 1 - Introduction

(a)Blue-dimer complex. (b)Ru(Hbbp) complex.

(c)Meyer’s Ru complex. (d)Ru(bda) type complex.

Figure 1.5:Ruthenium complexes, from dinuclear to single-site catalysts.

Artificial molecular water oxidation catalysts (WOCs) were designed with the idea that accommodating multiple metal centers was fully required to achieve the 4 stepwise PCET process. It was only in 2005, that Thummel group reported^57,58 a proof that four electron oxidation from H₂O could be possible on a single-site metal complex. The ruthenium metal center was coordinated with the tridentate 2,6-di(1,8-naphthyridin-2-yl)pyridine polypiridil ligand exhibiting uncoordinated nitrogen atoms. Those nitrogen atoms interact with aqua ligands through hy-drogen bonds stabilizing the aquo complex (Figure 1.5 [c]). In 2008 Meyer and co-workers reported⁵⁹a complete mechanistic study that demonstrated the mononuclear nature as a catalyst. The suggested catalytic cycle follows a WNA

1.4 - Water Oxidation Catalysts (WOC) 19 mechanism. The reaction starts with a Ru^II-OH₂species and oxidizes to Ru^V=O in acidic conditions, containing a high electrophilic oxo-group able to react with a water molecule coming from the solvent, generating the corresponding hydroper-oxide species Ru^III-OOH. In addition, another oxidation is required to activate the catalyst and form Ru^III-OO, as rate-determinant step (RDS) to finally evolve O₂ and restart the catalytic cycle.

Interestingly Ru complexes containing the equatorial bda^{2 –} ([2,2’-bipyridine]-6,6’-dicarboxylate) ligand are the fastest molecular water oxidation catalysts described so far in the literature^60–62(Figure 1.5[d]). Two crucial features of Ru−(bda) is the capacity of the carboxylate moieties present in the ligand to form intramolecular hydrogen bonds with the active Ru−OH group at different oxidation states, which is beneficial for the catalysis in terms of both thermodynamics and kinetics. Other key feature is the ligand capacity to stabilize Ru at high oxidation states via the anionic character of the carboxylate groups and also via the formation of seven-coordinated beyond oxidation state IV. This stabilization reduces the overpotential for the catalytic reaction up to 740 mV in comparison to other mononuclear Ru complexes with the classical octahedral coordination.

Many works have been devoted to molecular mononuclear metal complexes, not only with ruthenium, but also with iridium based complexes, as well as earth abundant metals such as cobalt,^63–66copper^67,68 or iron.^69,70

Iridium Molecular Complexes

A significant discovery on the field of H₂O oxidation catalysis occurred when the first iridium mononuclear cyclometalated complex was able to accomplish H₂O oxidation^71,72 (Figure 1.6-left). This complex employs a cyclometalated phenylpyridine (ppy) as bidentate ligand, and takes advantage of the strong iridium-carbon bond to afford the oxidative hard conditions required for H₂O oxidation (TON 2490^nO₂/ncatand TOF 0.0041S⁻¹).

20 Chapter 1 - Introduction

Figure 1.6:Ir complexes, [Left] mononuclear and [Right] dinuclear catalysts.

Theoretical studies of the iridium mononuclear complex were performed, with the aim to bring out a feasible mechanism through the (WNA) of H₂O on the generated iridium-oxo species to form the desired O−O bond.⁷³The reaction follows an acid/base mechanism, in which the O−O bond is generated by the attack of water to the electrophilic Ir−−O moiety. The proton released by water is transferred to different acceptors, depending on the nature of the ancillary ligand.

Such ligand depends on the pH conditions and can range from OH₂, OH or O^{– 2}. With the OH₂aquo ligand, the Ir−−O plays a double role by both making the O−O bond and accepting the proton in the WNA. In contrast, with OH^– ligand, the Ir−−O makes the O−O bond and the hydroxo ligand takes the proton. Finally, with the O^{– 2}oxo ligand one, makes the O−O bond whereas the other accepts the proton. Such ligand cooperates to facilitate the reaction by generating the nucleophilic hydroxide anion. As the basicity of the ligand increases from OH₂to O^{– 2}, the energy barrier decreases. This feature suggests that water oxidation catalysts may be improved with ligands able to act as internal bases.

In general, dinuclear metal complexes (Figure 1.6-right) can be more stable than mononuclear ones, because it capacity to distribute the oxidizing equiva-lents over several metal centers. The synthesis of dinuclear iridium complexes such as [(cod)(Cl)Ir(µ-bpi)Ir(cod)]⁺, where (bpi) is a (pyridin-2-ylmethyl)(pyridin-2-ylmethylene)-amine was achieved. The first Ir center is coordinated to the imine nitrogen throughσ-coordination, which activates the imine scaffold towardsη² coordination to the other Ir center.

1.4 - Water Oxidation Catalysts (WOC) 21 Although the extensive amount of metal complex able to perform the H₂O oxi-dation, the main structures and their performance have been discussed. This acquired knowledge can be extrapolated to the rest of molecular catalyst being useful to understand heterogeneous and colloidal catalysts.